The present invention relates generally to the field of active noise control. More specifically, the present invention is an actuator for use in completely-in-the-canal active noise control systems inside the ear.
Noise pollution is not merely an irritating aspect of urban life. In certain environments, such as airports, factories, and military operations, noise pollution poses a serious hazard to the hearing of those exposed. As a consequence, means have been devised to both passively and actively reduce the noise exposure of individuals who must work in these environments.
The first commercial devices combined noise filtering electronics with earmuffs to passively reduce ambient noise while amplifying speech and other desired sounds. The external noise, however, was not actively reduced in these devices.
Active noise reduction (ANR) followed. Earmuffs were combined with analog feedback technology to “cancel” undesired audio signals. The basic components of an ANR system are a microphone that “hears” the sound levels received at the ear of a listener, electronics that process the sound signal from the microphone and relay a cancellation signal to an actuator, and the actuator that converts a cancellation signal into sound pressure and “adds” it to the received signal, which added sound combines with the existing ambient noise to reduce the overall noise level. It is the “summing” of those signals that represents the active cancellation.
The effectiveness of an ANR system is influenced by a number of parameters, including by way of example, the location of the audio sensors, the varied shape of the human ear, the lag between signal detection and the creation of the cancellation signal, and the nature of the actuator and acoustic space.
Passive earmuffs (that is, ear coverings without ANR) theoretically provide protection against mid-to-high frequency noise in the audio bandwidth, reducing noise levels by as much as 25 dB or more, but are less efficient at low frequencies. Further, earmuffs are generally heavy, not comfortable, and impractical in environments where the user is shaken (as in heavy vehicles, for example).
Earmuffs with ANR are more effective at low frequencies, typically 600 Hz and below. As the desired control frequency range increases, the acoustic field represented by the volume inside the earmuff becomes more difficult to model and control. Over a broader frequency spectrum, the number of acoustic modes within this acoustic field increases, making control more difficult.
One way to increase the controlled frequency spectrum and reduce the number of acoustic modes is to reduce the size of the acoustic field. Reduction of the acoustic field is accomplished by substituting an earplug for the earmuff. From a strict acoustic perspective, an active earplug placed in the ear canal should be able to control from low frequencies up to “high” frequencies (e.g., several kilohertz). However, the reduction in size of the ANR system leads to a number of significant design issues, particularly the design of the system component that converts electrical signals to sound pressure (referred to herein generically as an “actuator”).
A number of designs of earplugs with ANR are described in “An Active Noise Reduction Ear plug with Digitally Driven Feedback Loop,” by K. Buck, V. Zimpter and P. Hamery, a paper presented at Inter-Noise 2002, the International Congress and Exposition on Noise Control Engineering, Aug. 19–21, 2002 (herein, “Buck”). One such design used a walkman-type loud speaker and a Knowles miniature microphone (as used in hearing aids). According to Buck, the results were not impressive, largely due to the electroacoustic transfer function of the walkman-type loud speaker.
Buck also describes a piezoceramic actuator. A flat-plate type device exhibited an electroacoustic transfer function that was amenable to ANR applications. However, the pressure output of the flat-plate piezoceramic actuator was insufficient for ANR applications, particularly in a noisy environment. A tube-type piezoceramic actuator was also tested. Like the flat-plate design, the transfer function of the tube-type piezoceramic actuator was acceptable, but the prototype's electric energy conversion was too inefficient for commercial applications. Buck concludes that:                the main problem in designing ANR earplugs are the transducers [actuators]. If electrodynamic earphones shall be used, the amplitude phase relationship observed is often not minimum phase, and so, impedes on the bandwidth of the active attenuation. Piezoceramic devices show a good behavior as far as the transfer functions are concerned, however, the output levels that can be reached under realistic circumstances are still too low.        
Another paper, “Electroacoustic Design of an Active Earplug,” by Phillipe Herzog, a paper presented at Inter-Noise 2002, the International Congress and Exposition on Noise Control Engineering, Aug. 19–21, 2002 (herein, “Herzog”), also discusses the design of earplugs with ANR. Herzog comments on the design constraints posed by current choices for actuators:                The piezoelectric speaker would allow to use a simpler control filter, but still require expensive developments. An cheaper solution, requiring also a simple control filter, would be electret speaker, if a relatively low pressure is to be controlled. Conversely, the emergence efficient numerical control filters may allow us to use existing dynamic speakers. In any case, the maximum pressure inside the ear canal remains a critical criterion.        
Buck and Herzog both focus on the actuator as a weak link in the design of an effective earplug with ANR system. Notably, both papers discuss the use of filters as a means for compensating for the deficiencies of various existing actuators. Unfortunately, there are well-known limitations on the compensation that can be offered by the filter designs and this approach cannot correct the technical deficiencies associated with current actuator designs when used in an ANR earplug system.
Actuators useful for ANR earplug designs must be very small. Two choices are available. First, the actuator can be placed in the portion of the earplug that fits into the concha of the ear. The dimensions of this space depend on the user, with mean areas on the order of 13 mm×8 mm and mean depths of that space of approximately 3–4 mm. Second, the actuator can be placed completely-in-the-canal (CIC) where the dimensional constraints are even more stringent. From a size perspective, current actuator designs that could possibly be used for ANR earplugs are limited to hearing aid speakers and so-called microspeakers, such as those used in earbud products. The smallest available microspeakers are approximately 10 mm in diameter. Although the smallest available microspeakers may fit in some percentage of user's concha spaces, the ANR performance cannot optimally satisfy all of the ANR objectives when the speaker is in that location. Hence, there is a need for new voice coil ANR actuator designs that can be used in the CIC ANR application. That leaves only the subminiature hearing aid actuators that could possibly be fit into a CIC ANR earplug design.
In addition there are electroacoustic design limitations for existing hearing aid speakers and microspeakers that could be used in the CIC or concha earplug designs, respectively. Until now, actuators have been designed to fulfill the audio or hearing aid applications for which they were intended, and are generally not well suited for feedback active noise reduction. In order to optimize an actuator for in-the-ear ANR, the actuator dynamics, power handling capabilities, and dimensional sizing must be simultaneously considered.
A practical actuator for in-the-ear ANR should achieve a high percentage fit rate. For CIC designs, this consideration further restricts the size of the actuator. Measurements of a population of human ear canals reveal that 95% of the smallest ear canal diametrical dimensions are larger than approximately 5.5 mm. Allowing for an earplug wall thickness of approximately 1.5 mm results in a two-sigma width dimensional limitation of the actuator of 4 mm. Other measurements of the length of the ear canal reveal that approximately 95% of the population have an ear canal length of greater than 11 mm. This places a two-sigma length dimensional constraint on the actuator design. Ideally, the actuator could be made smaller and still satisfy the remaining goals of the design.
In addition to size constraints, the actuator has two more relevant constraints for use in active noise control. First, the sound power output of the actuator must be sufficient to accommodate the sound pressures required for control. This is an increasingly challenging goal as the ambient sound pressures to be controlled increase but the size of the actuator decreases to meet the geometrical constraints. To a first approximation, the sound pressure levels that can be created inside the occluded space of the ear canal are proportional to a volume change inside that occluded space. Larger volume changes are required to generate higher sound pressure levels. The volume change of the occluded space is equal to the displacement of the actuator times the actuator area. Therefore, the ANR application calls for very small profile actuators that still provide required volumetric changes with reduced diaphragm areas. Actuator design methods and embodiments for achieving this volume change are explained in the following paragraphs.
The remaining challenge for an ANR actuator is related to the frequency response of the device, as referred to a voltage (input) applied to the motor terminals and a microphone measurement of the sound pressure (output) produced in the earplug user's occluded ear canal space by the actuator. (Here, the “motor” of a speaker refers to the combination of the magnet and coil that together cause motion of the diaphragm through an electrical current running in the coil). It is desirable to construct an actuator that minimizes the occurrence of dynamic properties or resonances (a large pole-zero excess) leading to phase lag across the control bandwidth. More specifically, minimizing the amount of phase that is present in the actuator input-to-output frequency response is very desirable, and minimizing the number of dynamics is one way to accomplish this goal. A second way to achieve this goal is to construct an actuator that does not interrupt the linear systems theoretical property, referred to as collocation, where an alternating pole-zero pattern characterizes the transfer function of systems that satisfy collocation (See, Martin, G. D., “On the Control of Flexible Mechanical Systems,” PhD Dissertation, Stanford Univ, 1978). Collocation defined in this way ensures that the phase across the frequency band of interest will be minimized. Current designs from the audio community and the hearing aid community are not concerned with achieving this goal of minimum phase across the audio bandwidth because phase response is not important for either application.
The audio community most typically employs designs that are referred to as voice coil speakers. These consist of a coil of wire attached to a diaphragm, where the coil of wire is situated in a magnetic field. A typical voice coil speaker is illustrated in FIG. 1. Diaphragm 105 is suspended by its outer edge from frame 100 by suspension 110. The inner edge of diaphragm 105 is suspended from the frame 100 by spider 115. The center of the diaphragm 105 is attached to voice coil 120. The voice coil 120 is suspended in a magnetic field generated by magnet assembly 125. The speaker is characterized by the diameter “D” of diaphragm 105 and motor depth “M”, which is a measure of the greater of the magnet assembly 125 length or the length of the voice coil 120. When an electrical voltage is applied to voice coil 120, opposing magnetic fields induce diaphragm 105 to move. This diaphragm motion then displaces air particles generating sound pressure that is proportional to the applied voltage or applied current. The consumer and professional markets have not driven voice coil speaker technologies toward sizes that are small enough to fit in a human ear canal because until now there has not been a technical need.
As introduced above, ANR technology can benefit from decreased actuator size and by placing the actuator as deep as possible in the ear canal to minimize the volume change requirements from the speaker. Microspeakers used strictly for audio applications utilize a geometrical profile wherein the diaphragm diameter is equal to or perhaps larger than the depth of the speaker motor (defined to be the magnet and wound coil below the diaphragm). For the CIC application, this acoustical consideration is no longer the fundamental consideration, thereby relaxing the need for large diaphragm diameters and allowing new speaker designs with diaphragm to motor depth ratios below unity. Conventional voice coil designs employ a diameter to motor dimensional ratio of one or greater. As discussed above, the conventional microspeaker geometry is not amenable to ear canal because the diameters are too large for the CIC dimensions cited earlier. However, if the microspeaker were simply reduced proportionally to its conventional profile, the sound output power would not be high enough to enable effective active noise control. Another disadvantage of existing voice coil technologies is that the diameters that are available today are too large to fit in the ear canal and the diaphragm stiffness of such devices are marginal in being able to resist high acoustic load forces that are present when the device is used in a very small space. Although the smallest available microspeakers may fit in some percentage of user's concha spaces, the ANR performance cannot optimally satisfy all of the ANR objectives when the speaker is in that location.
The unique problem of fitting a new speaker design completely-in-the-canal also affects the speaker design from an acoustic perspective. It is well known in acoustic theory that a speaker can be approximated by a radiating piston whose efficiency of radiation into an ambient medium is proportional to the diameter of the piston. Therefore, in order to achieve satisfactory acoustic response at low frequencies (below 500 Hz), the speaker diameter must be an appreciable portion of the acoustic wavelength. Existing microspeaker dynamic voice-coil designs for audio applications have been selected using the conventional profile described above, where the speaker diaphragm is typically larger than the depth of the speaker motor (the greater of the magnet or voice coil dimension) and leads to acceptable radiation efficiency.
The hearing aid industry has almost exclusively employed balanced armature technologies to deliver high sound power in a small package. The modern balanced armature or reed driven devices employ a voltage driven coil that changes the polarity of an armature in the presence of a permanent magnetic field. When the polarity alternates with the alternating voltage, the armature moves toward or away from one of the permanent magnet poles. The armature is attached to a diaphragm via a very tiny rod. When the armature moves, the diaphragm moves and generates a change in volume. The front of the diaphragm is covered by a front enclosure that leads to a small port where the sound exits. The port and enclosure are designed into these devices to protect the diaphragm, provide a means to secure the diaphragm to the hearing aid housing, to change the load impedance on the diaphragm, and to provide a way to attach a tube to the actuator.
A typical balanced armature actuator is illustrated in FIG. 2. Diaphragm 205 is suspended by its outer edge from housing 200 by suspension 210. Diaphragm 205 is connected to armature 208 by coupling member 215. Voice coil 220 surrounds a portion of armature 208. The opposite end of armature 208 is suspended in a magnetic field generated by magnet assembly 225. Housing 200 covers diaphragm 205 creating enclosure 235. Sound exits through enclosure 235 through port 230.
Balanced armature manufacturers are currently motivated by the hearing aid industry and tailor their designs accordingly. Overall phase lag in a design is not important, whereas additional sound power output is important. By adding resonant dynamics, the sound power output of the balanced armature speaker designs are effectively increased at the expense of additional phase lag.
There are a variety of dynamic systems in the traditional balanced armature actuator that make it suboptimal for active control:                an acoustic system in front of the diaphragm that is separate from the occluded space environment;        a port that is used to connect the actuator to a tube in hearing aids that acts as a Helmholtz resonator that also adds additional dynamics in the control band;        vibrational modes of the diaphragm and reed itself;        the mass-spring-damper system of the moving driven diaphragm;        the dynamic system of the magnetically driven armature; and        a compressional mode of the rod connecting the armature and diaphragm.        
All of these dynamics present significant amounts of phase lag, or pole-zero excess, in the frequency response of the input voltage to output sound pressure of the actuator. The connectivity of the different dynamic systems also precludes the opportunity for collocated response as measured in the acoustic domain. The increased phase lag from the dynamics and non-collocation limits the amount of active noise control attenuation that can be achieved, and especially the bandwidth over which it can be achieved.
There are several significant disadvantages to this type of actuator design when employed with active noise control in a high noise environment. First, current state of the art designs have difficulty achieving the highest required sound pressure levels for ANR in very high noise fields while still fitting inside a human ear canal. Second, the balanced or reed armature design described above exhibits a mechanical resonance, speaker cabinet resonances, and other structural dynamic resonances associated with the armature's mechanical design. Because of the series and/or parallel connections of the different acoustic (cabinet) and mechanical (diaphragm/reed) system resonances, and their respective couplings to the acoustical field, all existing balanced armature actuators built for in-ear devices preclude the opportunity for collocated input-output dynamic response between the speaker driving voltage and a microphone placed in the acoustic field. This degradation of collocated response has negative impact on the phase lag of the frequency response, and hence on the effectiveness of feedback active noise control, as is well known by those versed in the art.
What is needed is an actuator that meets the geometrical constraints for CIC placement, that is able to effect volumetric changes in the resulting occluded space of the ear such that sound pressures in that space are sufficient for control, and that exhibits specific dynamic properties that lead to stable, high performance closed-loop ANR response.